Digital image sensing based upon solid state technology is well known, the two most common types of image sensors currently being charge coupled devices (CCD's) and complementary metal oxide semiconductor (CMOS) image sensors. Digital image sensors are incorporated within a wide variety of devices throughout the consumer, industrial and defense sectors, among others.
An image sensor is a device comprising one or more radiation sensitive elements having an electrical property that changes when radiation is incident thereupon, together with circuitry for converting the changed electrical property into a signal. As an example, an image sensor may comprise a photodetector that generates a charge when radiation is incident upon it. The photodetector may be designed to be sensitive to electromagnetic radiation in the range of (human) visible wavelengths, or other neighboring wavelength ranges, such as infra-red or ultra-violet for example. Circuitry is provided that collects and carries the charge from the radiation sensitive element for conversion to a value representing the intensity of incident radiation.
Typically, more than one radiation sensitive element will be provided in an array. The term pixel is used as a shorthand for picture element. In the context of a digital image sensor, a pixel refers to that portion of the image sensor that contributes one value representative of the radiation intensity at that point on the array. These pixel values are combined to reproduce a scene that is to be imaged by the sensor. A plurality of pixel values can be referred to collectively as image data. Pixels are usually formed on and/or within a semiconductor substrate. In fact, the radiation sensitive element comprises only a part of the pixel, and only part of the pixel's surface area (the proportion of the pixel area that the radiation sensitive element takes up is known as the fill factor). Other parts of the pixel are taken up by metallization such as transistor gates and so on. Other image sensor components, such as readout electronics, analogue to digital conversion circuitry and so on may be provided at least partially as part of each pixel, depending on the pixel architecture.
Image sensors of this type may be used for still image capture and for video capture. Even when an image sensor is primarily designed for still image capture, it is common for a video function to be provided for example to provide a viewfinder function in a screen of a digital camera.
Many light sources are not illuminated continuously. For example, fluorescent lamps may flicker with the source frequency. Also, other light sources such as LEDs may actually be pulsed such that they are only switched on for only a short time overall, possibly less than 20% of the time. This improves efficiency, while also making it easier to control brightness via control of the LED's duty cycle (the ratio of time on to the overall time). This pulsing, provided it is fast enough, cannot be resolved by human eyes and therefore the light emitted is perceived by humans as being continuous.
A problem exists where an image needs to be taken of the light source. The exposure time to capture the image may be very short such that there is at least a significant chance, and in the case of the LEDs discussed above a probable chance, that the image will be captured when the source is not actually illuminated, even though the image capture occurs when the LED is “switched on” such that it would appear illuminated to a person. In other words the illumination pulse and exposure time may not actually coincide. To compound this problem, the pulsing is not necessarily predictable and therefore the exposures cannot simply be timed to coincide with pulses.
There are many applications where accurate image capture of an illumination source is not only desirable but essential. One such application involves using sensors of various types at various locations in or on a vehicle such as an automobile. Image sensors may be provided to perform various functions based on detection of images inside the vehicle and outside the vehicle. These can include light detection systems, for example, brake light detection (brake lights now commonly comprise LEDs or other pulsed sources). It is clearly very important that such systems are able to accurately sense whether the light is active.
There is a need in the art to provide an image sensor that addresses one or more of the above issues.